<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <script>
    document.onclick=()=>{
        let  a = ()=>{
            alert(this);
        }
        let btn = document.getElementById('btn1');
        btn.onclick = a.bind(12);
    }
    </script>
  </head>
  <body>
        <input type="button" value="aaa" id="btn1">
  </body>
</html>
